It is well known in the art of fuel delivery systems for aircraft gas turbine engines to use a primary electronic fuel control system with a mechanical secondary or backup system for monitoring and scheduling fuel delivery to the engine. The fuel control has two operating modes. In the first or primary mode the electronic engine control (EEC) schedules fuel flow to the engine in conjunction with the hydromechanical unit (HMU). In the backup or secondary mode the fuel delivery to the engine is scheduled by the HMU only.
The problem arises in that the EEC and HMU schedule a different fuel flow in response to pilot input. This is due to the fact that the EEC monitors different parameters than the HMU and can set an optimum engine speed (fuel flow) to achieve a desired thrust level. The HMU sets an engine speed for a given power lever setting. The differences in commanded engine speed are especially critical during takeoff. If the fuel control should switch from the primary to the secondary control the change in commanded engine speed could result in an excessive increase or decrease in engine thrust.
There exists a need, therefore, for a fuel delivery system which minimizes changes in engine thrust when switching from a primary control to a backup or secondary control.